Comparative Description of the Microwave Surface Impedance of Nb, BaKBiO, and YBaCuO

Overview & Synopsis

Measurements of microwave impedance versus temperature, Zs(T), in Nb, Ba0.6K0.4BiO3, and YBa2Cu3O7-δ are given. The measurements were made by the “hot finger” cavity perturbation technique. The electromagnetic properties of Nb are in a good quantitative agreement with the isotropic s-wave BCS theory. This model can also describe the experimental Zs(T) curves in Ba0.6K0.4BiO3 crystals if the nonhomogeneous broadening of the superconducting transition is properly taken into account. The behavior of the microwave impedance of YBa2Cu3O7-δ disagrees with the isotropic BCS theory. This suggests a strong anisotropy of the order parameter in this compound. Since the controversy between s- and d-wave descriptions of YBa2Cu3O7-δ properties has not been resolved yet, we have checked a simpler strongly anisotropic s-wave model. Calculations by this model have been compared to measurements on YBa2Cu3O7-δ. With due account of the strong-coupling effects, two-band anisotropy, and impurity scattering in YBa2Cu3O7-δ, the agreement between the generalized BCS model and experimental data is surprisingly good.